And the racket path used by each player for the same forehand as shown above is interestingly the same: the racket for both Federer and Nadal are moving 18 degrees upward…"

18 degrees... do you realize how flat this is? And they're not hitting winners, they're rallying with their practice partner! The host did what none of you did and what people like Oscar will never do: he studied it with thousands of pictures taken from super-high speed videos.

Do you know how he compares these two forehands? If I recall, he numbers about 40 anatomical movements that both of these players share -- and some which aren't shared by most top pros.

Why does Nadal hits loopy shots and Federer hits mostly flat strokes?

"Nadal often swings on a much steeper upward path than Federer (especially on the clay) and this steeper path further amplifies the amount of topspin he generates by tilting his racquet face forward throughout his forward swing. Nadal’s steeper overall swing path (see below), combined with his extreme forward racquet tilt throughout his forward swing through impact, results in 25% higher topspin production, on average, compared to Federer, as well as a slightly higher launch angle off of the stringbed that creates a shot with a slightly higher trajectory."

Nadal's forehand is indeed swung at nearly 30 degrees here... If you understood what I have been saying, you know what happened: loads of spin, but also a very high ball.